The invention relates to a method of determining variations in the previously adjusted nominal distance between the facing surface of a colour selection electrode and a display window of a given nominal glass thickness of a colour television display tube.
The invention also relates to a device for carrying out this method.
Non-electric quantities, for example distances, can be measured electrically by means of capacitance determinations. Such measurements, however, are difficult to make in particular, in the case of small distances. In the book by Kautsch "Messelektronik nicht-elektrischer Grossen", volume 3, pp. 98, 99, the principle is explained of the measurement of a layer thickness of a dielectric and a formula is derived for the capacity of a capacitor which is filled with two dielectrics. Furthermore, it is known from the book by F. Kohlrausch "Praktische Physik", volume 2, page 237 to use for the accurate measurement of the dielectric properties of plate-shaped insulators a so-called screening capacitor in which one capacitor plate is a metal plate and the other one is a circular electrode surrounded by an annular screening electrode and a plate-shaped electrode surrounded by a screening electrode, respectively.
Furthermore, German Auslegeschrift No. 2,041,044 discloses a capacitor in which one capacitor plate is surrounded by a screening electrode and which is used for measuring very small capacitance variations.
As described in the above-mentioned book by Kautsch, the capacitance C between a measuring electrode and a metal plate is inversely proportional to the distance a between the measuring electrode and the metal plate. This means that a variation in the distance a also results in a capacitance variation because as a matter of fact it holds that: ##EQU1## where .epsilon. is the dielectric constant of the medium between the plates and F is the surface area of the measuring electrode. Thus by measuring the capacitance, the distance a is directly obtained. The measurement is the more accurate as the medium between the actual measuring electrode and the opposite electrode is more homogeneous. When a screening electrode is used which may consist, for example, of an annular thin metal plate, a substantially homogeneous measuring field is obtained. The distance between the measuring electrode and the screening electrode should be chosen to be as small as possible in order that no inhomogeneous peripheral disturbances can occur at said areas.
When the measuring space is filled with two different dielectrics formed from planeparallel plates, in which one dielectric has a dielectric constant .epsilon..sub.1 and a layer thickness a.sub.1 and the other dielectric has a dielectric constant .epsilon..sub.2 and a layer thickness a.sub.2, it holds for the overall capacitance that: ##EQU2## From this it follows that: ##EQU3## The distance a.sub.1 is then given by ##EQU4## Such a configuration of two dielectrics is encountered in the manufacture of colour television display tubes having a colour selection electrode which is positioned at a small distance from a glass display window. One dielectric is formed by the glass display window and the other dielectric is formed by the medium between the facing surfaces of the glass display window and the colour selection electrode. For true color reproduction it is important during manufacture of a colour television display tube to accurately establish whether the distance between the facing surfaces of the display window and the colour selection electrode corresponds to the previously adjusted nominal distance. When it is determined by means of a capacitance measurement, the distance in the above formula (3) is equal to a.sub.1. The glass thickness of the display window then is equal to a.sub.2. The distance a.sub.1 can be measured accurately only if the distance a.sub.2 is known accurately. However, in a display window of a colour television display tube, variations occur in the glass thickness, which in turn result in variations in the measured distance between the colour selection electrode and the display window. With .epsilon..sub.1 .apprxeq. 1 (air) and .epsilon..sub.2 .apprxeq. 7 (glass) a variation in the glass thickness of, for example, 1 mm causes an error of approximately 140 .mu.m in the measured distance between the colour selection electrode and the display window, as follows from the above formula (3). During the manufacture of a colour television display tube, however, a greater accuracy is required for determining the distance. It should be possible to establish deviations of approximately 30 .mu.m from the previously adjusted nominal distance between the colour selection electrode and the display window.